Mass Spectrometry in A-Level WJEC Chemistry | A-Level WJEC 化学：质谱考点精讲

📚 Mass Spectrometry in A-Level WJEC Chemistry | A-Level WJEC 化学：质谱考点精讲

Mass spectrometry is an indispensable analytical technique in A-Level WJEC Chemistry, used to determine the relative atomic masses of elements, the relative molecular masses of compounds, and to elucidate the structures of organic molecules. By converting a sample into gaseous ions and separating them according to their mass-to-charge ratio (m/z), a mass spectrometer produces a unique spectrum that acts like a fingerprint for the substance. Understanding how to interpret this spectrum – from the molecular ion peak to fragmentation patterns and isotopic abundances – is a core skill for any A-Level chemist.

质谱法是 A-Level WJEC 化学中不可或缺的分析技术，用于测定元素的相对原子质量、化合物的相对分子质量以及解析有机分子的结构。质谱仪将样品转化为气态离子，并按质荷比 (m/z) 进行分离，从而生成该物质特有的质谱图，犹如其指纹。理解如何解读谱图——从分子离子峰到碎片模式和同位素丰度——是每一位 A-Level 化学学习者必须掌握的核心技能。

1. What is Mass Spectrometry? | 什么是质谱法？

Mass spectrometry is a technique that measures the mass of individual atoms or molecules after converting them into ions. It can be used to identify unknown compounds, quantify known substances, and probe the structural features of molecules. In A-Level chemistry, its primary roles are to determine relative atomic masses of elements from isotopic abundances, determine relative molecular masses of organic and inorganic compounds, and provide structural information through fragmentation patterns.

质谱法是一种先将原子或分子转化为离子，再测量其质量的技术。它可用于鉴定未知化合物、定量已知物质以及探测分子的结构特征。在 A-Level 化学中，其主要用途包括：根据同位素丰度测定元素的相对原子质量，测定有机和无机化合物的相对分子质量，并通过碎片化模式提供结构信息。

2. Key Stages in Mass Spectrometry | 质谱分析的关键步骤

Mass spectrometry involves four key stages: vaporisation (if the sample is not already gaseous), ionisation, acceleration and deflection in a magnetic field, and detection. The sample is first vaporised and then ionised to form positive ions. These ions are accelerated by an electric field and passed through a magnetic field, where they are deflected according to their mass-to-charge ratio (m/z). Ions with a smaller m/z are deflected more, while heavier ions are deflected less. By varying the magnetic field strength, ions of different m/z values can be focused onto the detector sequentially, generating a mass spectrum.

质谱分析包括四个关键步骤：气化（如果样品不是气态）、电离、在磁场中加速和偏转，以及检测。样品首先气化，然后电离形成正离子。这些离子被电场加速后通过磁场，在磁场中根据其质荷比 (m/z) 发生偏转。质荷比较小的离子偏转更多，而较重的离子偏转较少。通过改变磁场强度，可将不同 m/z 值的离子依次聚焦到检测器上，从而生成质谱图。

The detector records the ion current for each m/z value, producing a spectrum of relative abundance against m/z. The most intense peak is called the base peak and is assigned a relative abundance of 100%. All other peaks are measured relative to this base peak.

检测器记录每个 m/z 值的离子流，生成相对丰度对 m/z 的谱图。最强峰称为基峰，设定其相对丰度为 100%。所有其他峰都相对于该基峰进行测量。

3. Electron Impact Ionisation | 电子轰击电离

In electron impact (EI) ionisation, the vaporised sample is bombarded with high-energy electrons (typically 70 eV). This knocks out an electron from the molecule, forming a radical cation, M⁺•. The molecular ion M⁺• often has excess internal energy, which causes it to fragment into smaller cations, radicals, and neutral molecules. EI produces extensive fragmentation, providing rich structural information but sometimes making the molecular ion peak very weak or even absent.

在电子轰击 (EI) 电离中，气态样品受到高能电子（通常 70 eV）的轰击。这会从分子中打出一个电子，形成自由基阳离子 M⁺•。分子离子 M⁺• 通常具有过剩的内能，导致其碎裂成更小的阳离子、自由基和中性分子。EI 产生大量碎片，提供丰富的结构信息，但有时会使分子离子峰非常弱甚至缺失。

EI is the most common ionisation method in standard organic mass spectrometry, and it is

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